Method of making a dielectric chuck

ABSTRACT

A method of making a dielectric chuck for securing a semiconductor wafer on a pedestal having multiple apertures for the introduction of cooling gas beneath the wafer. The wafer is held by electrostatic force against a laminate of an electrode layer sandwiched between two dielectric layers in accordance with the method, such that the laminate presents a planar surface to the wafer for a substantial distance beyond the outer edge of the electrode layer. The laminate construction method ensures that a large wafer area beyond the outer edge of the electrode is in contact with the laminate, to minimize cooling gas leakage near the edge, and provides a longer useful life by increasing the path length of dielectric material between the electrode layer and potentially damaging plasma material surrounding the chuck.

This application is a divisional application of prior U.S. applicationSer. No. 08/203,111, filed Feb. 28,1994.

BACKGROUND OF THE INVENTION

This invention relates generally to electrostatic chucks used to securesemiconductor wafers in processing chambers and, more particularly, tosignificant improvements that enhance wafer cooling and increase theuseful life of chucks of this type.

Electrostatic chucks are used in semiconductor processing chambers tohold individual semiconductor substrates or wafers on a chamberpedestal. An electrostatic chuck typically includes a dielectric layerand an electrode. A semiconductor wafer is placed in contact with thedielectric layer and a de voltage is applied to the electrode to createan electrostatic attractive force to grip the wafer to the chuck. Chucksof this type are particularly useful in vacuum processing environmentsin which there is insufficient differential pressure to hold the waferin position, or in which mechanical clamping of the wafer isundesirable. One processing environment in which the electrostatic chuckis widely used is the plasma etch process.

Although an electrostatic chuck may be formed using only a singledielectric layer and an electrode, a more typical configuration includesa thin laminate member having an electrode core, which is preferably athin copper layer, sandwiched between upper and lower dielectric layersor an organic material such as polyimide. A polyimide adhesive may beused to assemble these layers into a single laminate sheet, and toattach the laminate the chamber pedestal.

The upper and lower polyimide layers merge at the circumferential edgeof the electrode to prevent exposure of the copper electrode to theprocess chamber, which usually contains a plasma that would damage thecopper. In operation, the copper electrode is connected to a voltagesource and functions as an anode. The principles of operation ofelectrostatic chucks are well known and not critical to the presentinvention. It is sufficient to note that an electric field formed abovethe pedestal results in mutual attraction between the pedestal and asemiconductor wafer placed in contact with the laminate.

Many processes to which wafers are subjected result in the generation ofheat, and means must be provided to cool the wafer to an acceptableprocess temperature to prevent heat damage. In most electrostatic chucksof the prior art, helium gas is introduced beneath the wafer through acentral aperture in the pedestal, and is then distributed through apattern of grooves in the laminate on the pedestal. Various groovepatterns have been used in an effort to distribute the cooling gasuniformly across the wafer surface, which is generally circular inshape. A difficulty inherent to this cooling technique prevents uniformcooling near the wafer edge. If the grooves are continued all the way towafer edge, there is a high rate of gas leakage and an accompanyingreduction in the cooling effect, especially near the edge. In mostdesigns, the grooves are terminated before they reach the edge. In somedesigns, radial grooves adjoin secondary grooves near the edge, and thesecondary grooves may extend circumferentially for short distancesbefore terminating, or may take the form of branch grooves extending invarious directions before terminating. In all these designs, however,there is still undesirable overheating of the wafer near its peripheraledge.

Another drawback of electrostatic chucks of the prior art arises fromthe way the laminate of polyimide and copper is formed on the chuckpedestal. A first layer of polyimide is placed over the pedestal andthen the copper electrode is placed over the polyimide. Of course, thecopper layer cannot extend all the way to the edge of the polyimide,because the polyimide layers have to overlap and provide a seal aroundthe outer edge of the copper. When the second polyimide layer is placedover the copper, the total thickness of the laminate is smaller aroundthe polyimide edge (the thickness of two polyimide layers) than thetotal laminate thickness including the copper. Therefore, the laminatehas an annular step outside the outer diameter of the copper layer. Thisannular step has two detrimental effects on the performance of theelectrostatic chuck. First, because of the step there is only a verysmall region of the chuck providing contact with the wafer beyond theouter diameter of the copper layer. As a result, there is a highprobability of helium leakage at the wafer edge, and unwantedoverheating of the wafer edge region. A second effect of the annularstep in the laminate structure is that the outer edge of the copperlayer is insulated from the damaging chamber environment by only a verythin polyimide layer, having a thickness approximately the same as thatof the upper polyimide layer. Once this layer is eroded away by theprocess plasma in the chamber, the chuck has to be replaced. Polyimideand other organic materials have a relatively low tolerance for manyprocess gases and plasmas. Therefore, provision of a good insulatinglayer around the electrode is an important consideration.

It will be appreciated from the foregoing that there is a need forimprovement in electrostatic chucks. In particular what is needed is animproved chuck construction to enhance cooling near the peripheral edgeof the wafer and to lengthen the useful life of the chuck. The presentinvention satisfies this need, as will be apparent from the followingsummary.

SUMMARY OF THE INVENTION

The present invention resides in an electrostatic chuck providinggreatly improved heat transfer between the wafer and the chuck,especially at the edge portion of the chuck, by distributing cooling gasthrough the chuck at multiple points across the area of the chuckpedestal. In addition, the chuck of the present invention presents aplanar surface to the wafer all the way to the edge of a laminatemounted on the chuck pedestal, thereby further improving cooling of edgeportions of the wafer and increasing the useful life of the chuck.

Briefly, and in general terms, the electrostatic chuck of the inventioncomprises a pedestal for supporting a workpiece; a laminate including aninsulated copper electrode, attached to the pedestal, wherein theworkpiece is electrostatically clamped to the laminate upon applicationof a voltage to the copper electrode; and a plurality of holes extendingup through the pedestal and the laminate, to carry a gas for cooling theworkpiece from beneath, many of the holes being positioned near theouter periphery of the workpiece. The pedestal also includes a coolinggas reservoir formed beneath the pedestal and extending across all ofthe holes, and a cooling gas supply port into the reservoir. Preferably,the holes are tapered to a smaller diameter at their top ends.

More specifically, the laminate includes a first insulating layer; acopper electrode layer formed over the first insulating layer; and asecond insulating layer formed over the copper electrode and mergingwith the first insulating layer around edges of the copper electrodelayer, to provide insulation of the copper from a process environment inwhich the electrostatic chuck is installed. In accordance with oneaspect of the invention, the second insulating layer presents an uppersurface that is substantially planar over the entire width of thelaminate. Good contact is thereby maintained with the workpiece beyondthe outer edge of the copper electrode. Moreover, the copper electrodeis widely separated from the effects of the process plasma, to increasethe useful life of the electrostatic chuck.

To achieve this planar upper surface, the pedestal has a recess formedin its upper surface. Therefore, the first insulating layer when placedover the pedestal conforms with the recess. The copper electrodesubstantially fills the recess, presenting a composite upper surfacethat is substantially planar and is then covered by the secondinsulating layer.

The electrostatic chuck of the invention may also be defined in morespecific terms as including a pedestal for supporting a semiconductorsubstrate; and a laminate including an insulated copper electrodeattached to the pedestal, wherein the substrate is electrostaticallyclamped to the laminate upon application of a voltage to the copperelectrode. The laminate includes a first dielectric layer, a copperelectrode layer over the first dielectric layer, and a second dielectriclayer over the copper electrode and merging with the first dielectriclayer around edges of the copper electrode layer, to provide insulationof the copper from a process environment in which the electrostaticchuck is installed. The second dielectric layer presents an uppersurface that is substantially planar over the entire width of thelaminate, so good contact is maintained with the substrate beyond theouter edge of the copper electrode. Further, the copper electrode iswidely separated from the effects of the process plasma, to increase theuseful life of the electrostatic chuck.

In terms of a novel method, the invention includes the steps of forminga shallow recess in the top of the pedestal, the recess having a depthequivalent to the thickness of the electrode; placing a first dielectriclayer across pedestal; placing the electrode layer over the firstdielectric layer, whereby the electrode layer fits within the shallowrecess; and placing a second dielectric layer across the firstdielectric layer and the electrode layer, to form a relatively planarsurface. The electrode layer is then effectively insulated from theprocess environment by a relatively long insulation path, and the planarsurface ensures good contact with the substrate in a region outside theouter edge of the electrode.

It will be appreciated from the foregoing that the present inventionrepresents a significant advance in the field of electrostatic chucks.In particular, the invention provides a chuck in which overheating ofthe edge portion is virtually eliminated and temperature gradients aredramatically reduced. Moreover, the configuration of the laminate memberof the chuck ensures better contact with wafer around its outer regionand a longer useful life for the chuck.

Other aspects and advantages of the invention will become apparent fromthe following more detailed description, taken in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an electrostatic chuck of the prior art,showing radial cooling grooves diverging from a central cooling supplypassage in the chuck pedestal;

FIG. 2 is cross-sectional view of the prior art chuck of FIG. 1, takensubstantially along the line 2--2, and also showing a semiconductorwafer supported on the chuck;

FIGS. 3A and 3B are diagrammatic plan views of electrostatic chucks ofthe prior art, depicting two alternative cooling groove configurations;

FIG. 4 is an enlarged fragmentary cross-sectional view of anelectrostatic chuck of the prior art, depicting a portion of a laminatepolyimide and copper layers formed over the chuck pedestal;

FIG. 5 is a plan view of an electrostatic chuck in accordance with thepresent invention, showing multiple cooling holes formed in the chuckpedestal;

FIG. 6 is a cross-sectional view of the electrostatic chuck of FIG. 5,taken substantially along the line 6--6;

FIG. 7 is a fragmentary cross-sectional view of an electrostatic chuckof the prior art, taken substantially along the line 7--7 of FIG. 1 andshowing how the laminate of polyimide and copper layers is formed on thechuck pedestal; and

FIG. 8 is fragmentary cross-sectional view similar to FIG. 7, butshowing a structure in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in the drawings for purposes of illustration, the presentinvention is concerned with improvements in electrostatic chucks. Asshown in FIGS. 1 and 2, a typical electrostatic chuck includes apedestal, indicated by reference numeral 10, on which is mounted alaminate 12 of multiple layers (to be described). For simplicity, thelaminate 12 is shown in FIG. 2 as a single line, its detailed structurebeing shown in FIG. 4. A central aperture 13 in the pedestal 10 extendsup through the laminate 12 and communicates with multiple grooves 14extending radially across the laminate 12 from the central aperture. Thepedestal 10 is shown as being secured to a pedestal base 15 by screws16. An annular insulating collar 17 is mounted on the pedestal. The topof the pedestal 10 and the laminate 12 are generally circular in shape,to match the outline of a semiconductor wafer 18 placed on the laminate.As best shown in FIG. 4, the laminate 12 includes a lower polyimidelayer 20, a copper electrode layer 22 and an upper polyimide layer 24.The upper polyimide layer 24 merges and seals with the lower one 20around the edges of the copper layer 22, both at the periphery of thelaminate 12 and along each of the grooves 14, one of which is shown inFIG. 4.

FIGS. 3A and 3B show alternative groove configurations that have beenused in the past. In the configuration of FIG. 3A, each of the radialgrooves 14 merges into an arcuate or part-circular groove 14' extendingaround the laminate 12 near its periphery. In the configuration of FIG.3B, each of the radial grooves 14 adjoins additional groove branches14", which extend toward the laminate periphery in different directions.These and other groove configurations are all attempts to providesufficient cooling gas near the edge of the wafer 18 without placinggrooves so near to the edge that excessive cooling gas leakage occurs.All of these configurations suffer from a common deficiency: overheatingof the wafer edge region. In all cases there is little heat transfer andelectrostatic clamping pressure in the area of the grooves and there arelarge temperature gradients at the wafer edge.

Another significant disadvantage of electrostatic chucks of the priorart is illustrated in FIG. 7, which depicts an edge region of thelaminate 12 and wafer 18 in detail. The laminate 12 is formed by firstplacing the lower polyimide layer 20 on the pedestal 10, then placingthe copper electrode layer 22 over the lower polyimide layer, andfinally forming the upper polyimide layer 24 over the first two layers20 and 22. Necessarily, this construction leaves an annular gap 30between the upper polyimide layer 24 and the wafer 18 because thethickness of the two polyimide layers is less than the combinedthickness of the two polyimide layers and the copper layer. Thislaminate construction has two detrimental effects. First, because thewafer 18 is not supported by the laminate 12 in the region of the gap30, there is only a very small region of wafer-to-laminate contactbeyond the outer diameter of the copper layer 22. There is noelectrostatic chucking force developed beyond the outer edge of thecopper layer 22. Therefore, cooling helium gas supplied at a pressure ofseveral torr, up to 20 torr, leaks into the process chamber over thissmall region. This leakage of the cooling gas results in lower gaspressures and higher temperatures near the wafer edge. The size of thecopper electrode 22 is limited by the size of the pedestal 10 and by therequirement for minimum overlap (approximately 1.5 mm) of the polyimidelayers 20 and 24.

The second detrimental effect of the annular gap 30 is that, because theprocess plasma is present in the gap, the minimum insulation of thecopper provided by the laminate is the distance 31 between the outeredge of the copper layer 22 and the annular gap 30. In known chucks,this distance is close to the thickness of one of the polyimide layers,which is approximately 0.025 to 0.050 mm. This small insulationthickness results in a relatively short useful life for the chuck.

In accordance with one important aspect of the invention, and as shownin FIG. 8, the laminate 12 is formed in such a way as to present aplanar upper surface extending well beyond the outer diameter of thecopper layer 22. Specifically, the copper layer 22' is recessed into thelower polyimide layer 20' and these two layers present a planar surfaceon which the upper polyimide layer 24' is formed. One way to recess 32the copper layer 22' by an appropriate amount is to first form acircular recess 32 in the top surface of the pedestal 10, by machining acounterbore of diameter selected to be greater than the diameter of thecopper electrode 22' by approximately 0.1 to 0.2 mm. The depth of therecess is the same as the thickness of the copper electrode 22', i.e.,approximately 0.040 mm. Then the lower polyimide layer 20' fills therecess in the pedestal 10 and leaves a corresponding recess in the uppersurface of the lower polyimide layer, into which the copper layer 22' isplaced.

This construction overcomes the disadvantages of the prior art discussedwith reference to FIG. 7. In particular, because the laminate 12presents a planar upper surface well beyond outer diameter of the copperlayer 22', the outer regions of the wafer 18 have a greater area ofcontact with the laminate and edge leakage is reduced. Also, the annulargap 30 of FIG. 7 is eliminated in FIG. 8 and the minimum insulationthickness between the copper layer 22' and the plasma is increased toapproximately 1.5 mm, i.e., by a factor of thirty or more. Therefore,the useful life of the chuck is significantly extended and its processperformance is improved.

Another important aspect of the invention is its use of multipleapertures to introduce cooling gas on the lower face of the wafer 18.This is best shown in FIGS. 5 and 6, in which a pedestal 34 is providedwith a coolant reservoir 36 extending across the bottom of the pedestaland formed in part by a circular plate 38 secured to the pedestal byscrews 40. Cooling gas is introduced into the reservoir 36 through asingle passage 42, which may be at one edge of the reservoir, andO-rings 44 seal the reservoir. There are multiple holes 46 formedthrough the pedestal to its upper surface. Holes in correspondingpositions (not shown) are also formed in the laminate 12, which, forsimplicity, is shown in this figure as a single line. The holes 46 areformed similarly to the way grooves were formed in prior art structures.(See FIG. 4.) There may be as many as 100-200 holes, many of which arepositioned close to the edge of the laminate 12. The holes 46 are smallin diameter; for example 0.25-0.50 mm, and are preferably tapered to asmaller diameter at the upper or outlet ends.

A test of a prototype of the invention in a tungsten etchback processresulted in an overall reduction of wafer temperature by approximately10° C. and a reduction of the wafer temperature at its peripheral edgefrom 80° C. to approximately 60°-65° C. Moreover, the temperaturegradient between the wafer center and its edge was reduced from 15°-20°C. to 6°-10° C. In contrast with difficulties associated withoverheating in electrostatic chucks of the prior art, repeatable andacceptable process results were obtained using the chuck of theinvention.

It appears from the test results on the prototype of the invention thata helium leak at the wafer edge that might have been regarded asexcessive in chucks of the prior art is not detrimental to the processif the electrostatic chuck provides sufficient heat conductance and asufficient helium supply to feed the leak at the wafer edge. Forexample, edge leakage in chucks of the prior art are approximately0.2-1.0 sccm (standard cubic centimeters per minute), but in the chuckof the invention the presence of leakage in the range 2-5 sccm at 9 torrpressure showed no detectable effect on the process. Most of the leakagewas from holes near the wafer edge. Blocking these holes reduced leakageby a factor of approximately ten, i.e. to 0.4-0.6 sccm, but caused waferedge overheating and loss of selectivity.

It will be appreciated from the foregoing that the present inventionrepresents a significant advance in the field of electrostatic chucks.In particular, the electrostatic chuck of the invention provides moreefficient cooling over the entire wafer surface, especially theperipheral edge, improved clamping effect and wafer contact at the edge,and increased useful product life. It will also be appreciated that,although a specific embodiment of the invention has been described indetail for purposes of illustration, various modifications may be madewithout departing from the spirit and scope of the invention.Accordingly, the invention should not be limited except as by theappended claims.

We claim:
 1. A method of making a dielectric chuck having a pedestal anda laminate member with an insulated electrode, the method comprising thesteps of:forming a shallow recess in the top of the pedestal, the recesshaving a depth equivalent to the thickness of the electrode; placing afirst dielectric layer across pedestal; placing the electrode layer overthe first dielectric layer, whereby the electrode layer fits within therecess; placing a second dielectric layer across the first dielectriclayer and the electrode layer, to form a relatively planar surface;wherein the electrode layer is effectively insulated from the processenvironment by a relatively long insulator path.